1. Field of the Invention
This invention relates to a head shifting apparatus, and more particularly to a head shifting apparatus for shifting a head intermittently or stepwise relative to a recording medium.
2. Description of the Prior Art
Most of the recording or reproducing apparatus of the type using disc-shaped recording media, for example, magnetic sheets, employ a shifting mechanism for advancing the recording or reproducing head track by track in a radial direction of the recording media, which comprises a step motor and a shifting screw connected to the output shaft of the motor. For making determination of a basic increment of shifting motion by the step motor, from a point of view of improving the precision accuracy the shifting of the head in one step for the required length of advance, for example, equal to each track pitch, is inferior to that through a plurality of steps, for example, in the case of the step motor being of 4-phase type, four steps for the same for the same track pitch, as is well known in the art. Therefore, in the prior art apparatus, the use of multi-step shifting mechanisms prevails.
Another important aspect of this type recording or reproducing apparatus is that regardless of whether in the recording or reproducing mode, whatever means must be provided for detecting when the head is set at a predetermined reference position relative to the recording medium. As is the common practice in the art, it is through the same number of steps for each track pitch when the head is shifted in a direction to the limit position that the head is then shifted in the opposite direction track to track. With such shifting control means, however, as will be fully described later, when the shifting steps of the head are disturbed by external disturbance, the head cannot be brought into accurate alignment with the reference position. This leads to a poor compatibility of the instruments.